aldiwhore, I'm not having a go at you, but I am quite sensitive about this. Parents wanting to make costumes and have their kids dress up is something that actually caused problems at our school. Part of the reason behind it also was that we have some massively competitive parents, who aren't short of money, so they were spending a fortune to try and have the "best" costume. We then have the complete opposite end of the scale with students who are from very low socioeconomic backgrounds, or parents who are not interested in their education, and it became quite the social divide.
Whilst some opportunities can be tied to learning, the majority of them are not. Wear your favourite football team colours, pink day and St Patricks day for example are some that have fortunately been wiped out here. You can have plenty of fun in the classroom without expecting parents to have to try and organise costumes for a day that is not tied to learning. If they're going to spend time doing something for school, then please read with your children or supervise their homework, not try and create an alien mask out of tin foil that is better than the one that every other student has.
I also put a stop to "Sorry Day" when someone tried to have the bright idea of everyone dressing up in aboriginal clothing. Ummm no. Not everything needs to be an opportunity to play dress ups. We are able to have plenty of fun an educational activities tied to the day without offending the local indigenous population.
The students are allowed to wear plain clothes on sports days in their house team colours (red, blue, green and yellow, so pretty easy for parents, and we keep some coloured zinc and hair ribbons around for those who arrive without anything), and we have one plain clothes day for charity per year. The only one I really support is when we travel to a historical site (Grade 1, 3 and overnight for grade 6) when they all get to dress up in period costume (and yes, even boring old me does too) and spend the day taking part in a traditional school classroom and households. Now that is a valuable learning opportunity. The costumes are supplied by site and returned at the end of the day.
For Book Week each class gets to vote on their favourite book, and we tie our lessons in to that book and then we do have a lunchtime fancy dress parade on the Friday, but costumes are made in class during the week and tied to other learning activities (usually just masks for the little ones, the older ones get a bit more creative). Parents aren't required to then have to spend money or do any extra work, and it avoids the competitive parents, because the kids all have the same materials and make the costumes themselves, which is far more valuable than having a parent putting it together for them.